: CD8+ T cells play a critical role in defense against viral, intracellular bacterial, and protozoan infections. To confer protective immunity, vaccines against these agents need to elicit potent CD8+ T cell memory. Despite recognition of their importance in vaccinations, the underlying mechanisms in the generation of memory T cells remains poorly understood. The magnitude of T cell memory is dependent upon the extent of clonal expansion and, the subsequent death of activated antigen-specific T cells. Understanding the generation of memory T cells is contingent upon elucidating the mechanisms that regulate proliferation and apoptosis of activated CD8+ T cells in vivo in a physiological setting. Our long term goal is to understand the molecular and cellular basis of CD8+ T cell memory. We have initiated studies investigating the role of TNF receptors (TNFRs) in regulating the generation of CD8+ memory T cells using the lymphocytic choriomeningitis virus (LCMV) model in mice. Preliminary studies have revealed that there is a dramatic enhancement in the number of LCMV-specific memory CD8+ T cells in TNFR I-deficient mice, as compared to wild type (+1+) mice. The goal of this application is to understand the mechanisms by which TNFRs regulate generation of memory CD8+ T cells. We hypothesize that TNFR I signaling downregulates CD8+ T cell responses in vivo and determines the magnitude of T cell memory. The objectives of this application are three fold: First, to elucidate the mechanistic basis of downregulating CD8+ T cell responses by TNFRs, by examining the effect of TNFR deficiency on the proliferation and apoptosis of LCMV-specific CD8+ T cells in vivo. Second, to dissect the importance of TNFR signaling on CD8+ T cells (direct effects) vs. non-CD8+ T cells (indirect effects) in regulating the generation of LCMV-specific memory CD8+ T cells, by using bone marrow chimeras, CD4-deficient, and B cell-deficient mice. Third, to examine the role of TNFRs on the functional attributes of LCMV-specific CD8+ T cells in vitro and in (increased number of antigen-specific T cells) and qualitative (heightened sensitivity) differences in memory T cells. Our preliminary studies show that loss of TNFR I leads to increased number ("quantity") of memory CD8+ T cells. We will examine for qualitative differences between TNFR-deficient and +/+ LCMV-specific memory CD8+ T cells by comparing the activation thresholds to produce cytokines as a function of antigen concentration, CD8 requirement, and time. The function of TNFR-deficient memory CD8+ T cells will also be tested in vivo based on their ability to confer protective immunity against (i) a persistent LCMV infection and (ii) CD8+ T cell-mediated immunopathology. Despite several lines of evidence of a suppressive role for TNF in T-cell-mediated autoimmunity, the underlying regulatory mechanisms are not well understood. The proposed experiments in this application will provide critical information towards (i) development of effective vaccines; (ii) understanding the pathogenesis of autoimmune disorders, and (iii) formulating immunotherapies against immune-mediated diseases.